1. Field of the Invention
The invention relates to search units used in nondestructive ultrasonic testing. In particular, the invention relates to inspection wheel assemblies which house multiple ultrasonic transducers and are adapted to ride on the surface of a rotating cylindrical member to test for defects, for example, longitudinal and transverse defects.
2. Description of the Prior Art
The use of ultrasonic testing techniques, and specifically of ultrasonic crystals, for detecting discontinuities in metal products is a common mode of nondestructive testing. The crystals employed are typically piezoelectric crystals made of a material such as quartz. These crystals produce ultrasonic vibrations in response to a voltage of appropriate frequency impressed upon the crystal. When inspecting a tubular product for internal flaws using a reflection method, the crystal is maintained in a position relative to the surface of the product to transmit a short duration sonic wave pulse into the product at an angle such that a defect or discontinuity will cause the waves to be reflected to the crystal and produce a voltage response in the crystal. Since the crystal is de-energized immediately following the pulsed emission of a wave, reflected waves are received during de-energized periods and hence the reflected waves will produce a discernible signal which may be monitored, for example, on a cathode ray tube or a strip chart recorder. Pulse repetition rates of between 60 and 2000 pulses per second are employed for various types of inspections.
Typically, an ultrasonic inspection device will be calibrated using a standard identical to the goods being inspected. The standard may have one or more discontinuities of known magnitude so that the response of the device to known imperfections may be ascertained, and standards for accepting or rejecting the inspected goods may be established.
Ultrasonic inspection techniques are most typically employed at the site of manufacture of the articles being inspected. Thus, plate or tubular goods are typically inspected at the manufacturing plant using techniques which are well known in the art. However, the on-site inspection of new and previously used tubular goods presents different and unique problems.
In well drilling operations, drill pipe failures can be a costly and time-consuming occurrence. Washouts or drill string breakage can occur frequently if a drill pipe with sufficiently serious imperfections is employed. Most frequently such failures result from internal flaws in the tubular goods being used. Confronted with such a failure, it becomes necessary to trip the pipe out of the borehole to replace the failed joint. In the case of drill string breakage, it is also necessary to fish the parted portion of the string from the borehole before drilling can be recommenced. Hence, the value of an efficacious method of inspection, particularly for monitoring wall thickness and detecting internal flaws in drill pipe, is obvious.
An apparatus and method for ultrasonic inspection of tubular goods employed in the drilling of oil and gas wells is disclosed in U.S. Pat. No. 4,020,688, commonly assigned with this application to W. C. Lamb. According to this patented apparatus and method, drill pipe, well casing or tubing may be inspected as it is being tripped into or out of a well borehole, i.e., while the tubular member is in a substantially vertical mode. The apparatus includes a number of search units, preferably wheel search units, which house transducers and are positioned at 90.degree. centers around the pipe being inspected. The beam spread of the search units determines whether the entire circumference of the pipe is being inspected as it passes through the apparatus.
A review of the art reveals that there is an acute need for an apparatus and method for inspection of both new and previously used oil field tubular goods while such tubular goods are being stored preparatory to downhole use.